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12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Parity Violating Electron Scattering:Recent Results and Future Prospects
Krishna KumarUniversity of Massachusetts, Amherst
Acknowledgement:E158 and HAPPEX Collaborations,
J. Erler, C. Horowitz, W.J. Marciano, M.J. Ramsey-Musolf, K. Paschke, J. Piekerewicz, M. Pitt, P. Souder
June 12, 2006Jefferson Lab Users Meeting
(Expanded version of CIPANP06 Plenary Talk)
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
• 1960s: An Electroweak Model of Leptons (and quarks)
– SU(2)L X U(1)Y gauge theory predicted the Z boson• 1973: antineutrino-electron scattering
– First weak neutral current observation
• Mid-70s: Does the Weak Neutral Current interfere with the Electromagnetic Current?
– Central to establishing SU(2)L X U(1)Y
Weak Neutral Current (WNC) Interactions
•Gargamelle observes one νμ e- event•First measurement of weak mixing angle
Parity is violatedParity is conserved
Consider fixed target electron scattering
Low energy WNC interactions (Q2<<MZ2)
Historical Context:
Z0
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Parity-Violating (PV) Electron Scattering
gunpolarizedtarget
- gV and gA are function of sin2θW
- β is a kinematic factor-Q2 is the 4-momentum transfer-gT affected by QCD physics
10−4 ⋅ Q2APV ~ 10−5 ⋅ Q2 to
g = gAegV
T +β gVegA
T
•Established experimental technique: δ(APV) < 10 ppm•Cleanly observed weak-electromagnetic interference•Parity Violation in Weak Neutral Current Interactions•sin2θW = 0.224 ± 0.020: same as in neutrino scattering
APV in Deep Inelastic Scattering off liquid Deuterium: Q2 ~ 1 (GeV)2
E122 at the Stanford Linear Accelerator Center (SLAC)
C.Y. Prescott et. al, 1978
20 GeV polarized electron beam on a 30 cm LD2 target
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
• Steady progress in technology• part per billion systematic control• 1% normalization control
2006: MeV to TeV Physics1970s
1980s
Recently completed
FutureOngoing
Parity-violating electron scattering has become a precision tool
Combined with judicious choices of kinematics and targets:•Many-Body Nuclear Physics•Nucleon Structure Physics•Valence Quark Physics•Search for New TeV Physics
Address fundamental physics issues over a range of energy scales
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Outline
• Strangeness in Nucleons – New Results from HAPPEX at Jefferson Lab– Status and Plans for further measurements
• The Neutron Skin of a Heavy Spinless Nucleus– A planned measurement of elastic WNC amplitude off 208Pb
• Low Energy Weak Mixing Angle Measurements– Final Result from the E158 Experiment at SLAC– Possible New Measurements at Jefferson Lab and an LC
• PV Deep Inelastic Scattering at JLab at 11 GeV– Potential of precision studies of nucleon structure at high x
• Summary
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Nucleon Structure & StrangenessQCD is intractable at low Q2; what is its relationship to hadron structure?Why don’t sea quarks destroy Quark Model predictions?
Strange quarks are relatively light;What can we say about its role?
SU(3)f symmetry-breaking introduces uncertainties
Strange mass: 0-20%πN scattering:
What about the nucleon’s charge and magnetization distributions?
Neutrino deep inelastic scattering
Semi-inclusive DIS (HERMES)(Needs fragmentation functions)
∫ ±±+=Δ=Δ30.0
023.0
(syst)01.0(stat)03.003.0)("" dxxss
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
supports naïve picture
Elastic Electron Scattering 101
Nuclear charge distribution
Measure σ as a function of Q2
Neglecting recoil and spin:Obtain Fourier transform of charge distribution
Nucleon charge and magnetization distributions:GE(Q2), GM(Q2) GE
p(0) = 1 GMp(0) = +2.79 ≡ μp
electric and magnetic form factors GEn(0) = 0 GM
n(0) = -1.91 ≡ μn
r2 ρ
s(r)
r [fm]
charge distribution: +ve core,-ve fringe
neutron
Is this a valid picture?
Need flavor-separation of GE, GM
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Elastic Electroweak Scattering
GEs(Q2), GM
s(Q2)
p
AMEF AAAQGAσπα
++⎥⎦
⎤⎢⎣
⎡−=
24
2
APV for elastic e-p scattering:
Forward angle Backward angle
( ) eA
pMWA
ZM
pMM
ZE
pEE GGAGGAGGA '2sin41 , , εθτε −−===
Z0
Kaplan & Manohar (1988)McKeown (1989)
Helium: Unique GE sensitivityDeuterium: Enhanced GA sensitivity
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
1. Skyrme Model - N.W. Park and H. Weigel, Nucl. Phys. A 451, 453 (1992).
2. Dispersion Relation - H.W. Hammer, U.G. Meissner, D. Drechsel, Phys. Lett. B 367, 323 (1996).
3. Dispersion Relation - H.-W. Hammer and Ramsey-Musolf, Phys. Rev. C 60, 045204 (1999).
4. Chiral Quark Soliton Model - A. Slivaet al., Phys. Rev. D 65, 014015 (2001).
5. Perturbative Chiral Quark Model - V. Lyubovitskij et al., Phys. Rev. C 66, 055204 (2002).
6. Lattice - R. Lewis et al., Phys. Rev. D 67, 013003 (2003).
7. Lattice + charge symmetry -Leinweber et al, Phys. Rev. Lett. 94, 212001 (2005) & hep-lat/0601025
R.L. Jaffe, Phys. Lett. B 229 (1989) 275
Λ
K+s s
Q2 ~ 0.1 (GeV)2
How Big Are GEs, GM
s?Various theoretical estimates:•Vector Meson Dominance Models•Quark models•Dispersion Theory•Lattice Gauge theory•Chiral-Quark Soliton Model
)0( 2 =≡μ QGsMs
Little theoretical guidance on Q2 dependence
Experimental determination of non-zero Gs is unambiguous
Q2 ~ 0.1 GeV2
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Overview of Experiments
GMs, (GA) at Q2 = 0.1 GeV2
SAMPLE
HAPPEX GEs + 0.39 GM
s at Q2 = 0.48 GeV2
GEs + 0.08 GM
s at Q2 = 0.1 GeV2
GEs at Q2 = 0.1 GeV2 (4He)
open geometry, integrating
A4
GEs + 0.23 GM
s at Q2 = 0.23 GeV2
GEs + 0.10 GM
s at Q2 = 0.1 GeV2
GMs, GA
e at Q2 = 0.1, 0.23, 0.5 GeV2
Open geometry
Fast counting calorimeter for background rejection
Electron Beam
LH2 Target
SuperconductingCoils
Particle Detectors
G0
GEs + η GM
s over Q2 = [0.12,1.0] GeV2
GMs, GA
e at Q2 = 0.23, 0.62 GeV2
Open geometry
Fast counting with magnetic spectrometer + TOF for background rejection
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Status as of 2005
Over the past three years:New data from A4, G0 and HAPPEX
•Add SAMPLE & HAPPEX-He•Multiple constraints at Q2 ~ 0.1 GeV2
•Would imply 5-10% contribution to magnetic moment from strange quarks (50-100% of isoscalar magnetic moment)
Q2 ~ 0.1 GeV2
Forward angle e-p data
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Experimental Technique
Compton1.5-3% systContinuous
Møller2-3% syst
Polarimeters
High Resolution SpectrometerS+QQDQ 5 mstr over 4o-8o
Target400 W transverse flow20 cm, LH220 cm, 200 psi 4He
Cherenkovcones
PMT
PMTElastic Rate:1H: 120 MHz4He: 12 MHz
100 x 600 mm
12 m dispersion sweeps away
inelastic events
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
New Preliminary HAPPEX Results
Asy
mmet
ry (pp
m)
Slug
Hydrogen
Araw correction ~11 ppb
Slug
Asy
mmet
ry (pp
m) Helium
normalization error ~ 2.5%
APV = -1.60 ± 0.12 (stat) ± 0.05 (syst) ppmA(Gs=0) = -1.640 ppm ± 0.041 ppm
HydrogenSystematic control ~ 10-8
APV = +6.43 ± 0.23 (stat) ± 0.22 (syst) ppmA(Gs=0) = +6.37 ppm
HeliumNormalization control ~ 2%
Helicity Window Pair Asymmetry
Hydrogen
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
A4, G0, SAMPLE
HAPPEX
Implications and OutlookForward angle e-p data
•Approved program well-matched to ultimate sensitivity of the technique•Models dealing with “sea” properties are extremely challenging•Experimentally, a 20-year old quest nearing completion•Ultimate insight: unquenched Lattice QCD calculations with light chiral quarks
•Rapid variation at low Q2 unlikely•Await backward angle measurements from A4, G0•Deuterium running will provide constraints on GA•One high precision point at Q2~0.6
Q2 ~ 0.1 GeV2
G0 Status report by D. Gaskell(Parallel Session I) GM
s = 0.28 +/- 0.20
GEs = -0.006 +/- 0.016
~3% +/- 2.3% of proton magnetic moment
(<20% of isoscalar magnetic moment)
~0.2 +/- 0.5% of electric distributionHAPPEX-only fit suggests something even smaller:
GMs = 0.12 +/- 0.24
GEs = -0.002 +/- 0.017
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
An Alternate Fit to Data Q2 ~ 0.1 GeV2
Leinweber et al, Phys. Rev. Lett. 94, 212001 (2005) & hep-lat/0601025
Young et al, nucl-ex/0604010•Fit to all proton data up to Q2 ~ 0.3 GeV2
•Specific choices of Q2 dependence•Similtaneously fit axial form factor
Parallel Session IR. Young
•Central value and error of calculation•Details of fit
D. Toublan•Heavy vs light sea quark contribution to the magnetic moment
Zhu et. al.
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Talk by M. PrakashTomorrow morningConstrain
neutron star crust thickness
Probing Neutron-Rich Matter•The proton distribution of heavy nucleus: mapped via electron scattering•The neutron distribution:
•probed with hadrons•highly model-dependent•neutron “skin” ~ 0.1 - 0.3 fm?
•Neutron density a fundamental observable:•Impacts a variety of physics
•208Pb neutron skin and neutron star crust made of similar material• At what radius does transition from liquid to non-uniform matter take place?•Mean Field theory predicts correlation between neutron star transition density and 208Pb neutron skin
208PbRp ~ 5.5 fm
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
PREx at Jefferson Lab
A technically demanding measurement:
•Rate ~ 2 GHz•Separate excited state at 2.6 MeV•Stat. Error ~ 15 ppb•Syst. Error ~ 1 to 2 %
δ(APV) ~ 3% δ(Rp-Rn) ~ 1% Q2 ~ 0.01 GeV2 APV ~ 0.5 ppm
•Tight control of beam properties•New “warm” septum•High power Lead target•New 18-bit ADC•New radiation-hard detector•Polarimetry upgrade
QpEM ~ 1 Qn
EM ~ 0
QpW ~ 1 - 4sin2θWQn
W ~ 1
γ
Constrain neutron halo for Atomic Parity Violation Expts
Tentatively scheduled to run in 2008
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Beyond Standard Model @ Low Q2
•Precise predictions @ 0.1%•Indirect access to TeV scale
•World electroweak data has marginal χ2, but no discernable pattern•Data used to put limits on energy scale of new physics effects
•Parity-conserving contact interactions probed at 10-20 TeV level•Parity-violating contact interactions probed at few TeV level
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
The SLAC E158 Experiment
APV ∝ me Elab (1− 4sin2 ϑW )APV ≈ −4 ×10−9 × Ebeam × Pbeam
e- e- e
e
e
ee
-
-
-
-- e-
γZγZ 00
y-Violating Left-Right Asymmetry In Fixed Target Møller Scatt
Goal: error small enough to probe TeV scale physics
•Berkeley•Caltech•Jefferson Lab•Princeton•Saclay
•SLAC•Smith •Syracuse•UMass•Virginia
8 Ph.D. Students60 physicists
E158 Collaboration
2001: Engineering run2002-2003: Physics runs2004: First PRL2005: Final result
LH24-7 mrad
N+,N-BEAM TARGET
DETECTOR 2 GHz scattered Møller rate
154 cm targetL ~ 1038 cm-2s-1
45 GeV, 48 GeV85% longitudinal polarization
Precision beam monitoring Novel small-angle spectrometer
Radiation-hard, ultra-quiet detector
End Station A at SLAC
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
SLAC E158 Main Result
sin2θeff = 0.2397 ± 0.0010 ± 0.0008
APV = (-131 ± 14 ± 10) x 10-9
* Limit on ΛLL ~ 7 or 16 TeV
* Limit on SO(10) Z’ ~ 1.0 TeV
* Limit on lepton flavor violating coupling ~ 0.01GF
Phys. Rev. Lett. 95 081601 (2005)
End of the SLAC Fixed Target Program
6σ
•Czarnecki and Marciano•Erler and Ramsey-Musolf•Sirlin et. al.•Zykonov
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
The Average: sin2θw = 0.23122(17)
Weak Mixing Angle at HIGH Q2
⇒ mH = 89 +38-28 GeV⇒ S = -0.13 ±0.10
ALR AFB (Z→ bb)
sin2θw = 0.2310(3)↓
mH = 35 +26-17 GeVS= -0.11 ± 17
sin2θw = 0.2322(3)
↓mH = 480 +350-230
GeVS= +0.55 ± 17
Rules out the SM!Rules out SUSY!
Favors Technicolor!
Rules out Technicolor!Favors SUSY!
(also APV in Cs) (also Moller @ E158
3σ apart
•Tevatron & LHC will make some improvements on MW•sin2θW improvements at hadron colliders very challenging•Must wait for “Giga-Z” option of ILC or Neutrino Factory
W. Marciano, CIPANP06EW & BSM Session
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
•Comparable to single Z pole measurement: shed light on disagreement•Best low energy measurement until ILC or ν-Factory•Could be launched ~ 2012
Future Possibilities (Purely Leptonic)ν-e in reactor
Kurylov, Ramsey-Musolf, Su
95% C.L.JLab 12 GeVMøller
Does Supersymmetry (SUSY) provide a candidate for dark matter?•Neutralino is stable if baryon (B) and lepton (L) numbers are conserved•B and L need not be conserved (RPV): neutralino decay
can test neutrino coupling: sin2θW to ± 0.002
Møller at 11 GeV at Jlab sin2θW to ± 0.00025!Λee ~ 25 TeV reach!Higher luminosity and acceptance
JLab e2e @ 12 GeV
e.g. Z’ reach ~ 2.5 TeV
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Ultrahigh Precision at ILC
(world average ~0.00016)
Measure contribution from scalars to oblique corrections
ALR and MW at future colliders:Systematics extremely challenging!
t
Z
H
b
new
physics
Critical crosscheck
Energy scale to 10-4, polarimetry to 0.15%
E158 LC
Energy (GeV) 48 250-500
Intensity/pulse 4.5 × 1011 14 × 1011
Pulse Rate (Hz) 120 120
Pe 85% 90%
Time (s) 4 × 106 2 × 107
ALR (ppm) 0.15 1-2
δALR (ppm) 0.015 0.008
δsin2(θW) 0.001 0.00008
K.K, Snowmass 96
• Fixed target has advantages for systematics• Could work with ILC “exhaust”beam
Møller scattering at the ILC
δmH
mH
≈10% for δ sin2 θW ≈ 0.00004
σ ∝1
Elab
Figure of Merit rises linearly with Elab
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Qweak at JLab APV in elastic e-p scattering
( )2 4 22 Q( 0 Q)2
Q4
pweak
FGA Q B Qπα
⎡ ⎤→ = − +⎣ ⎦Physics Asymmetry:
Region 3: VerticalDrift chambers
Region 2:Horizontal drift chamber location
Region 1: GEMGas Electron Multiplier
Quartz Cerenkov Bars(insensitive to non-relativistic particles)
Collimator System
Mini-torus
QTOR Magnet
Trigger Scintillator
Lumi Monitors
e- beam
•δ(APV) ~ 3%•δ(sin2θW) ~ ± 0.0007•Design under way•Data ~ 2009
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Future Measurements (Semi-leptonic)• NuTeV motivates closer look at lepton-quark WNC couplings• 4 model-independent e-q couplings to nail down•Implications for models of new TeV scale physics
NuTeV
C1i ≡ 2gAe gV
i C2i ≡ 2gVe gA
i
A
V
V
A
• C2i’s small & poorly known: difficult to measure in elastic scattering• PV Deep inelastic scattering experiment with high luminosity 11 GeV beam
(2009-10)
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
PV DIS at 11 GeV with an LD2 target
•6 GeV experiment launches PV DIS measurements at JLab•11 GeV experiment requires tight control of normalization errors•Important constraint should LHC see anomaly•Need to characterize nucleon structure at high-x to high precision
APV =GFQ2
2παa(x) + f (y)b(x)[ ] a(x) =
C1iQi f i(x)i
∑Qi
2 f i(x)i
∑
y ≡1− ′ E / E
b(x) =C2iQi fi(x)
i∑
Qi2 f i(x)
i∑
e-
N X
e-
Z* γ*
For an isoscalar target like 2H, structure functions largely cancel in the ratio:
a(x) =
310
(2C1u − C1d )[ ]+L
b(x) =
310
(2C2u − C2d ) uv (x) + dv (x)u(x) + d(x)
⎡
⎣ ⎢ ⎤
⎦ ⎥ +L
(Q2 >> 1 GeV2 , W2 >> 4 GeV2, x ~ 0.3-0.5)•Must measure APV to 0.5% fractional accuracy!•Luminosity and beam quality available at JLab
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
1% APVmeasurements
Charge Symmetry Violation (CSV) at High x: clean observation possible?δu(x) = up (x) − dn (x)δd(x) = d p (x) − un (x)
Precision High-x Physics with PV DIS
-0.01
-0.005
0
0.005
0.01
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
x
Q2 = 10 GeV2
xδuv
xδdv
QEDbagQEDbag
δAPV (x)APV (x)
= 0.3δu(x) −δd(x)u(x) + d(x)
Global fits allow 3 times larger effects
APV =GFQ2
2παa(x) + f (y)b(x)[ ]
a(x) =u(x) + 0.91d(x)u(x) + 0.25d(x)
•Allows d/u measurement on a single proton!•Vector quark current! (electron is axial-vector)
Longstanding issue: d/u as x→1
For hydrogen 1H:
Londergan & Thomas
•Direct observation of parton-level CSV: exciting!•Implications for high energy collider pdfs•Could explain significant portion of the NuTeV anomaly
Need 1% APVmeasurement at x ~ 0.75
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
A Vision for Precision PV DIS Physics• Hydrogen and Deuterium targets• Better than 2% errors
– It is unlikely that any effects are larger than 10%
• x-range 0.25-0.75• W2 well over 4 GeV2
• Q2 range a factor of 2 for each x
– (Except x~0.75)• Moderate running times
•CW 90 µA at 11 GeV•40 cm liquid H2 and D2 targets•Luminosity > 1038/cm2/s
•solid angle > 200 msr•Count at 100 kHz• online pion rejection of 102 to 103
Goal: Form a collaboration, start real design and simulations, and make pitch to US community at the next nuclear physics long range plan (2007)
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Summary•New HAPPEX results on nucleon neutral weak form factors:•Helium: Gs
E = +0.004 ± 0.014(stat) ± 0.013(syst) (Q2 = 0.077 GeV2)•Hydrogen: Gs
E+0.088GsM = +0.004±0.011(stat)±0.005(syst)±0.004(FF)
•Final measurements to be completed within two years
•A clean measurement of the neutron’s skin in 208Pb: implications for neutron star formation and properties
•E158 has carried out a precision measurement of sin2θW•APV: -131 ± 14 ± 10 ppb•Running of weak mixing angle established at 6σ•sin2θeff = 0.2397 ± 0.0010 ± 0.0008•New constraints on TeV scale physics
•Future experiments could improve sensitivity by ~ 2 to 6•An “ultimate” measurement could be done at an LC
•New era of PV DIS measurements with JLab 12 GeV upgrade
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Search for CSV in PV DIS
Sensitivity will be further enhanced if u+d falls off more rapidly than δu-δd as x → 1
•measure or constrain higher twist effects at x ~ 0.5-0.6•precision measurement of APV at x ~ 0.7 to search for CSV
Strategy:
•u-d mass difference•electromagnetic effects
•Direct observation of parton-level CSV would be very exciting!•Important implications for high energy collider pdfs•Could explain significant portion of the NuTeV anomaly
up (x) = dn (x)?d p (x) = un (x)?
For APV in electron-2H DIS: δAPV (x)APV (x)
= 0.28 δu(x) −δd(x)u(x) + d(x)
δu(x) = up (x) − dn (x)δd(x) = d p (x) − un (x)
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
11
110
)(32)(3
1
)(31)(18
1)(21
==
===
↓−↑−
↓−↑+↑=↑
SS
SSS
uuduud
uduuduudup
Proton Wavefunction (Spin and Flavor Symmetric)
d(x)/u(x) as x→1
SU(6):d/u~1/2
Valence Quark: d/u~0Perturbative QCD: d/u~1/5
APV =GFQ2
2παa(x) + f (y)b(x)[ ]
a(x) =u(x) + 0.91d(x)u(x) + 0.25d(x)
•Allows d/u measurement on a single proton!•Vector quark current! (electron is axial-vector)
PV-DIS offhydrogen
Longstanding issue in proton structure
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
PV DIS and Nucleon Structure• Analysis assumed control of QCD uncertainties
– Higher twist effects– Charge Symmetry Violation (CSV)– d/u at high x
• NuTeV provides perspective
– Result is 3σ from theory prediction– Generated a lively theoretical debate– Raised very interesting nucleon structure issues:
cannot be addressed by NuTeV• JLab at 11 GeV offers new opportunities
– PV DIS can address issues directly• Luminosity and kinematic coverage• Outstanding opportunities for new discoveries• Provide confidence in electroweak measurement
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
E158 Analysis
electron flux
Basic Idea:
: quartz: copper
light guide
PMTshielding
air
Radial and azimuthalsegmentation
•Corrections for beam fluctuations•Average over runs•Statistical tests•Beam polarization and other normalization
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Run 1: Spring 2002Run 2: Fall 2002Run 3: Summer 2003
1020 Electrons on Target
Run 1: Apr 23 12:00 – May 28 00:00, 2002Run 2: Oct 10 08:00 – Nov 13 16:00, 2002Run 3: July 10 08:00 - Sep 10 08:00, 2003
Physics Runs
45 GeV: 14.0 revsg-2 spin precession
48 GeV: 14.5 revs
Data divided into 75 “slugs”: - Wave plate flipped ~ few hours - Beam energy changed ~ few days
APV Sign Flips
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Raw Asymmetry Statistics
Ai − A σ i
Ai − A σ i
σi ≈ 200 ppm σi ≈ 600 ppbN = 818N = 85 Million
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Final Analysis of All 3 Runs
45 GeV: 14.0 revsg-2 spin precession
48 GeV: 14.5 revs
APV = (-131 ± 14 ± 10) x 10-9
APV ≈ −1×10−7 × Ebeam × Pbeam × (1− 4sin2 ϑ W )≈ 250ppb
Phys. Rev. Lett. 95 081601 (2005)
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Electroweak Physics
sin2θW = e2/g2
→ test gauge structure of SU(2)×U(1)
3%
•Czarnecki and Marciano•Erler and Ramsey-Musolf•Sirlin et. al.•Zykonov
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Backgrounds & NormalizationIntegrating calorimeter:
background dilutions and asymmetries must be separately measured or bounded.•Elastic and inelastic e-p scattering and radiative tail•High energy pions•High and low energy photons•Neutrons•Synchrotron radiation
•Beam polarization measured using polarized foil target- Same spectrometer used with dedicated movable detector
•Energy scale and spectrometer alignment to determine <Q2>•Linearity of PMTs
Largest systematic errors: •Inelastic ep: -22 ± 4 ppb•Beam polarization: 0.89 ± 0.04
Total dilution: 9.3% in Run I, 7.6% in Run II & II
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Summary of Corrections
Correction fbkg σ(fbkg) Acorr (ppb) σ(Acorr) (ppb)Beam first order - - -10 1
Beam higher orders - - 0 3Beam spotsize - - 0 1
Transverse asymmetry - - -4 2High energy photons 0.004 0.002 3 3Synchrotron photons 0.002 0.001 0 1
Neutrons 0.003 0.001 -1 1ep elastic
ep inelastic0.0560.009
0.0070.001
-7-22
14
Pions 0.001 0.001 1 1
TOTAL 0.075 0.008 -40 6• Scale factors:
• Average Polarization 89 ± 4% New “NLC” cathode ! • Linearity 99 ± 1%• Radiative corrections: 1.01 ± 0.01
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
“ep” Detector Data
•Radiative tail of elastic ep scattering is dominant background•8% under Moller peak•Additional 1% from inelastic e-p scattering•Coupling is large: similar to 3 incoherent quarks: 0.8 x 10-4 x Q2
•Background reduced in Run II & III with additional collimation
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Transverse Asymmetry
Observe ~ 2.5 ppm up-down asymmetry w/ horizontal polarization
First measurement of single-spin transverse asymmetry in e-e scattering.
Flips sign at 43 GeV
Asymmetry vs φ
Two-photon exchange QED effect
Ebeam 46 GeV
φ
Theory References:1. A. O. Barut and C. Fronsdal, (1960)2. L. L. DeRaad, Jr. and Y. J. Ng (1975)3. Lance Dixon and Marc Schreiber (2004)
smA e
Tα
∝
for Møller scattering at 46 GeV
= −3.5 ppm • sin φ
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Form Factors
NFM
qiFNNuuNJ
Nqq
EM
⎥⎥⎦
⎤
⎢⎢⎣
⎡+== ∑ γ
νμνγ
μμμ
σγγ 21 2
Q
γγγγγγ τ 2121 FFGFFG ME +=+=Adopt the Sachs FF:
sME
dME
uMEME GGGG //// 3
131
32
−−=γ
sMEW
dMEW
uMEW
ZME GGGG /
2/
2/
2/ sin
341sin
341sin
381 ⎟
⎠⎞
⎜⎝⎛ −−⎟
⎠⎞
⎜⎝⎛ −−⎟
⎠⎞
⎜⎝⎛ −= θθθ
NC probes same hadronic flavor structure, with different couplings:
GZE/M provide an important new benchmark for testing
non-perturbative QCD structure of the nucleon
(Roughly: Fourier transforms of charge and magnetization)
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Charge Symmetry
One expects the neutron to be an isospin rotation of the proton*:
snME
spME
unME
dpME
dnME
upME GGGGGG ,
/,/
,/
,/
,/
,/ ,, ===
Gγ,pE,M
GsE,M
GuE,M
GdE,MGγ,n
E,MCharge
symmetryGΖ,p
E,M<N| sγμ s |N>
GnE,M
GpE,M
GsE,M
Shuffle
Well Measured
( )As
MEn
MEp
MEFZ
LR
LRPV GGGGQG
M
MMA ,,,F
2 ///
2
2 πασσσσ
γ
γ −=∝+−
=
sME
dME
uME
pME GGGG ///
,/ 3
131
32
−−=γs
MEu
MEd
MEnME GGGG ///
,/ 3
131
32
−−=γ
* See B. Kubis & R. Lewis nucl-th/0605006 & Randy Lewis’ talk at this meeting
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Beam Position Corrections, HeliumRaw Left Asymmetry
Raw Right Asymmetry Corrected Right Asymmetry
Corrected Left Asymmetry
ppm
ppm
Beam Asymmetries
Energy: -3ppb
X Target: -5 nm
X Angle: -28 nm
Y Target :-21 nm
Y Angle: 1 nm
Total Corrections:
Left: -370 ppb
Right: 80 ppb
All: 120 ppb
ppm
ppm
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Beam Position Corrections, HydrogenX Angle BPM
Energy: -0.25 ppb
X Target: 1 nm
X Angle: 2 nm
Y Target : 1 nm
Y Angle: <1 nm
Surpassed Beam Asymmetry Goals for Hydrogen Run
Corrected and Raw, Left arm alone,
Superimposed!ppm
micro
n
Total correction for beam position asymmetry on Left, Right, or ALL detector: 10 ppb
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Compton PolarimetryHydrogen: 86.7% ± 2%
Helium: 84.0% ± 2.5%
Preliminary
Preliminary
Continuous, non-invasive
Here : Electron Detector analysis
Cross-checked with Møller, Mott polarimeters
also: independent electron analysis
Helium ran with lower beam energy, making the analysis significantly more challenging.
New developments in both photon and electron analyses in preparation: anticipate <2% systematic uncertainty
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Miscellany
• Backgrounds: Bryan Moffit’s talkDilutions: 2.2% (4He) 0.8% (1H)
Systematic 60 ppb (4He) 16 ppb (1H)
• Q2 & effective kinematics: �Q2 < 1.0% Bryan Moffit’s talk
• Two-photon exchange corrections: small Marc Vanderhaeghan’s talk (no explicit correction made)
• Transverse asymmetry: measured directly in dedicated runs, �cancels in left-right sum;Systematic: 4 ppb (1H) 8 ppb (4He) Lisa Kaufmann’s talk
• Electromagnetic Form Factors: use Friedrich & Walcher parameterization, Eur. Phys. J. A, 17, 607 (2003), and BLAST data for GE
n
• Axial Form Factor: highly suppressed for 1H (not present for 4He)• Vector Electroweak Radiative Corrections: Particle Data Group
• Blinded Analysis
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
4He: Nuclear Effects
O+ � O+ T=0 transition
• Any one-body electroweak operator O: (eg Fetter and Walecka)
<J,T|O|J,T> = � �J,T (�,�’) <�’|O|�> |�> = complete set
one-body density matrix element single-particle matrix element (nuclear structure) (nucleon structure)
• Asymmetry involves ratio of weak/EM matrix elements (GEs and GE
T=0); Single term in J,T in transition; O same in weak and EM except for couplings
� same one-body density matrix elements in numerator/denominator � nuclear structure cancels, only nucleon form factors remain
This result is EXACT, if :
– 4He g.s. pure isospin state: Ramavataram, Hadjimichael, Donnelly PRC 50(1994)1174– No D-state admixture: Musolf & Donnelly PL B318(1993)263– Meson exchange corrections small: Musolf, Schiavilla, Donnelly PRC 50(1994)2173
• Nuclear effects all << 1%, no explicit correction made.
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
False Asymmetries 48 ppb
Polarization 192 ppb
Linearity 58 ppb
Radiative Corrections 6 ppb
Q2 Uncertainty 58 ppb
Al background 32 ppb
Helium quasi-elastic background
24 ppb
Total 216 ppb
Error Budget-Helium
False Asymmetries 103 ppb
Polarization 115 ppb
Linearity 78 ppb
Radiative Corrections 7 ppb
Q2 Uncertainty 66 ppb
Al background 14 ppb
Helium quasi-elastic background
86 ppb
Total 205 ppb
False Asymmetries 43 ppb
Q2 Uncertainty 12 ppb
Polarization 23 ppb
Linearity 15 ppb
Radiative Corrections 7 ppb
Al background 16 ppb
Rescattering Background 32 ppb
Total 63 ppb
Error Budget-HydrogenFalse Asymmetries 17 ppb
Q2 Uncertainty 16 ppb
Polarization 37 ppb
Linearity 15 ppb
Radiative Corrections 3 ppb
Al background 15 ppb
Rescattering Background 4 ppb
Total 49 ppb
2005 2004
2005 2004
HAPPEx
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
HAPPEX-II 2005 Preliminary Results
A(Gs=0) = +6.37 ppm
GsE = 0.004 ± 0.014(stat) ± 0.013(syst)
A(Gs=0) = -1.640 ppm ± 0.041 ppm
GsE + 0.088 Gs
M = 0.004 ± 0.011(stat) ± 0.005(syst) ± 0.004(FF)
HAPPEX-4He:
HAPPEX-H: Q2 = 0.1089 ± 0.0011 (GeV/c)2
APV = -1.60 ± 0.12 (stat) ± 0.05 (syst) ppm
Q2 = 0.0772 ± 0.0007 (GeV/c)2
APV = +6.43 ± 0.23 (stat) ± 0.22 (syst) ppm
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
EM Form Factors
Electromagnetic form factors parameterized as by:Friedrich and Walcher, Eur. Phys. J. A, 17, 607 (2003)
FF Error
GEp 2.5%
GMp 1.5%
GEn 10%
GMn 1.5%
GA(3) -
GA(8) -
GEn from BLAST:
Claimed uncertainty at 7-8%
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
BackgroundDedicated runs at very low current using track reconstruction of the HRS
Dipole field scan to measure the probability of rescattering
inside the spectrometer
AcceptanceRolloff
HeliumHelium QE in detector: 0.15 +/- 0.15%Helium QE rescatter: 0.25 +/- 0.15%Al fraction: 1.8 +/- 0.2%
Hydrogen:Al fraction 0.75 +/- 25 %Hydrogen Tail + Delta rescatter: <0.1%
Total systematic uncertainty contribution ~40 ppb (Helium), ~15ppb (Hydrogen)
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
Determining Q2
• Central scattering angle must be measured to δθ < 0.5%• Asymmetry distribution must be averaged over finite
acceptance
( )⎭⎬⎫
⎩⎨⎧
++++
−−−
= 222
2
)()()()(sin41
24 pM
pE
sM
nM
pM
sE
nE
pE
WF
PV GGGGGGGGQGA
τετεθ
πα
Asymmetry explicitly depends on Q2:
( )θcos122 −′= EEQ %12 <QδGoal:
Q2 measured using standard HRS tracking package, with reduced beam current
12 June 2006 Parity Violating Electron Scattering: Recent Results and Future Prospects
A Simple Fit (for a simple point)Simple fit:GEs = r_s*τGMs = mu_sIncludes only data Q2 < 0.3 GeV2
Includes SAMPLE constrainted with GAtheory and HAPPEX-He 2004, 2005
G0 Global error allowed to float with unit constraint
Nothing intelligent done with form factors, correlated errors, etc.
Quantitative values should NOT be taken very seriously, but some clear, basic points:
• The world data are consistent.
• Rapid Q2 dependence of strange form-factors is not required.
• Sizeable contributions at higher Q2 are not definitively ruled out. (To be tested by HAPPEX-III, G0 and A4 backangle.)
Preliminary
Recommended